Hive Mapper part is not getting started

Dear all,
I've mapped the cassandra column family with hive external table using DSE3.0.When I ran the query for certain predicate like select count(column1),column2 and then group by based on column2 my mapper part is executed completedly but the reducer part is not getting started. It keeps on running for 12hrs even for less then 25k keys.The whole ecosystem is running in the standalone machine.Does it not started due to heap overflow or some other problem can be exists.